A misfire code is set because one of three things is missing or not right: spark, gas, or compression. You checked compression and it's good, equal to the other cylinders? And there is spark on #8? That leaves a fuel problem, then. How does the spark plug look on #8- if the appearance is white, almost a blistered look to the bottom of the plug, that is an indication of lean running- not enough gas in the mixture at that cylinder. You may have an injector problem on #8.

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A misfire at idle that smooths out at cruise is ALMOST ALWAYS caused by a vacuum leak. Take a hard look at your fuel trims at idle. If the Bank 2 Long Term Fuel Trim is more than + a couple points higher than the Bank 1 trim, your problem is most likely a vacuum leak. If the Fuel Trims even-out while holding the engine at 2000 RPM for about 20 seconds, then a vacuum leak is DEFINITELY your problem. I would suspect a leaking lower intake gasket leak because an upper plenum leak will normally affect all cylinders.

have the crank sensor check for erratic operation. if you have done all of that then there is nothing else. and they way the computer knows it is misfiring (if it is) false code maybe and could be bad crank sensor. good luck

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Related Questions:

What codes P0304 & P0308 ? This just means the PCM detected a misfire on those cylinders .an could be a few different things !
What ignition type does your vehicle have coil on plug or a coil pack with 8 plug wire towers ? If you have coil on plug ignition there are two wires going to the coils , one has battery + an the other is control from PCM . Check power first with volt meter or test light . Or use a spark tester on those two cylinders . No Spark Testing control testing

Code 0308 indicates a misfire was detected in cylinder 8. You say you checked the fuel injector and it is OK. Other probable causes include:
1. Control unit software in need of updating
2. Faulty PCM or ECM
3. Ignition system wires
4. Fuel system

THis is the same that P0308 fault code when "check engine" light turn on. A P0308 code means that the the car's computer has detected that one of the engine's cylinders is not firing properly. In this case it's cylinder #8.

Symptoms may include: the engine may be harder to start the engine may stumble / stumble, and/or hesitate other symptoms may also be present

A code P0308 may mean that one or more of the following has happened: Faulty spark plug or wire Faulty coil (pack) Faulty oxygen sensor(s) Faulty fuel injector Burned exhaust valve Faulty catalytic converter(s) Running out of fuel Poor compression Defective computer

Possible Solutions:If there are no symptoms, the simplest thing to do is to reset the code and see if it comes back.

If there are symptoms such as the engine is stumbling or hesitating, check all wiring and connectors that lead to the cylinders (i.e. spark plugs). Depending on how long the ignition components have been in the car, it may be a good idea to replace them as part of your regular maintenance schedule. I would suggest spark plugs, spark plug wires, distributor cap, and rotor (if applicable). Otherwise, check the coils (a.k.a. coil packs). In some cases, the catalytic converter has gone bad. If you smell rotten eggs in the exhaust, your cat converter needs to be replaced. I've also heard in other cases the problems were faulty fuel injectors.

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If there are no symptoms, the first thing to do is to reset the code, with a code reader, and see if it comes back. If there are symptoms such as the engine is stumbling or hesitating, check all wiring and connectors that lead to the affected cylinder and plug/coil assembly. Depending on how long the ignition components have been in the car, it may be a good idea to replace them as part of your regular maintenance schedule. I would suggest spark plugs, spark plug wires, distributor cap, and rotor (if applicable). Otherwise, check the coils (a.k.a. coil packs-COP-). In some cases, the catalytic converter has collapsed. If you smell rotten eggs in the exhaust, your converter may need to be replaced. I've also been told several times that the problem in this case could be a faulty injector at that cylinder 8.

The piston rings in your engine might be needed to be replaced already... go to a local shop with a compression tester.. have your engine tested for compression leaks.. if it does have compression leaks.. then you need to change piston rings. Note** white smokes = engine is burning oil..